1. Field of the Invention
This invention relates to a focal plane shutter for cameras, provided with one or two shutter blades.
2. Description of Related Art
In focal plane shutters for cameras, two types are known: those in which two blade chambers are provided between three plate members, called a shutter base plate, an intermediate plate, and an auxiliary base plate so that two shutter blades, called a first blade and a second blade are separately placed in the two blade chambers, and those in which a single shutter blades is placed in a blade chamber provided between the shutter base plate and the auxiliary base plate. The former focal plane shutters are adopted in digital cameras as well as in silver-halide film cameras, while the latter focal plane shutters are adopted only in digital cameras.
In either case of the focal plane shutters, the structure of the shutter blade is nearly the same and the shutter is constructed with a plurality of arms, one end of each of which is rotatably mounted to the shutter base plate, and at least one blade pivotally supported by the plurality of arms through a connection shank. A driving member is rotatably mounted to the blade-chamber-outside surface of the shutter base plate, and its driving pin is extended from an arcuate slot configured in the shutter base plate into the blade chamber and is connected to one of the arms. Thus, the shutter blade is constructed so that an opening and closing operation is performed by the reciprocating rotation of the driving member.
The driving member is rotated at a high speed in an exposure operation, and hence when the operation is completed, the driving pin is made to abut on a shock absorbing member made from elastic material and attached to one end of the slot and is braked by temporarily compressing the shock absorbing member so that the bound of the driving pin is favorably suppressed and the driving pin is stopped. The shape of such a shock absorbing member and a structure for attaching the member to one end of the slot are set forth in Japanese Patent Kokai No. 2001-133839.
As mentioned above, the shock absorbing member on which the driving pin of the driving member abuts when the exposure operation is completed, as disclosed in Kokai No. 2001-133839, is generally made from elastic material, such as butyl rubber, so as to have a planar shape like the letter “C”, and is configured with two arcuate overhang sections of nearly the same shape (flange portions in Kokai No. 2001-133839) at both end of the shutter base plate along its plate thickness on the periphery. The shock absorbing member is compressed and deformed in its entirety and is admitted to the slot. The shock absorbing member is restored to its original shape at one end of the slot in a longitudinal direction, and is mounted in a state where the edge of the slot of the shutter base plate is held between the two overhang sections. Hence, in a state where the shock absorbing member is mounted, the two overhang sections protrude from both surfaces of the shutter base plate.
However, the protrusions of the overhang sections of the shock absorbing member from the surfaces of the shutter base plate cause the possibility that other constituent members mounted to the shutter base plate interfere in the operation. This brings about considerable design constraints. For example, when the overhang sections of the shock absorbing member protrude from the shutter base plate into the blade chamber, the shutter must be designed so that the constituent members of the shutter blade are prevented from colliding with the overhang sections during the operation. In particular, a big problem is that the head of the connection shank used to pivotally support the blade with respect to the arms collides with the overhang sections. Thus, in order to avoid this problem, it is conceivable that the thickness of the shutter base plate is reduced for a portion corresponding to the mounting part of the shock absorbing member (a part held between the two overhang sections) so that the overhang sections of the shock absorbing member do not protrude from the surfaces of the shutter base plate in a mounted state of the shock absorbing member.
However, even in the mounting structure mentioned above, the problem is not completely solved. That is, as is well known, the shutter blade vibrates in a direction perpendicular to a working plane (along the optical axis) during the operation. Thus, in this mounting structure, the head of the connection shank is brought into sliding contact with the shock absorbing member by the vibration of the shutter blade during the operation, and thereby the shock absorbing member is also compressed in the direction of the thickness of the shutter base plate. When such sliding contact is repeated for a long term, the overhang sections of the shock absorbing member cease to be restored to the original shape, partly due to the fact that the overhang sections are thin parts. Consequently, the top of a sliding contact part (an arcuate peripheral edge) is raised to project into the blade chamber, and the operation of the shutter blade is made unstable. Since the head of the connection shank is not to abut on the shock absorbing member as in the driving pin but to come into sliding contact therewith, there are problems that the shock absorbing member is subjected to wear, and a shock absorbing function and strength for mounting are degraded.